The present invention relates generally to radar tracking systems, and more specifically the invention pertains to a waveform and signal processing technique that eliminates troublesome Range-Doppler ambiguities. This is brought about by radiating a large bandwidth, long time duration waveform. Of considerable importance is that the method and system employs a matched filter receiver for an optimum Signal-to-Noise Ratio. Time expansion correlation is used in the receiver to reduce the bandwidth to allow signal and data processing with present day analog and/or digital components.
Multioctave bandwidth radar systems present some advantages which are valuable for use with emerging stealth technology. More specifically, a radar system that can meet its search and track requirements while remaining undetected by the enemy, will be less vulnerable to attack. This waveform design has many additional advantages over conventional waveforms. This large bandwidth (high resolution) waveform is ideally suited for target identification applications. This waveform provides anti-jamming features and naturally provides for clutter suppression. Also, this waveform is ideally suited to stealth aircraft onboard radar applications. This technology can also be applied to sonar system as well as radar.
Ultra-wideband (UWB) radar systems are subject to a number of inherent problems which need resolution. Range-Doppler ambiguities are a potential problem for all radar systems, but present a particular problem to UWB systems as described below.
When a radar target is illuminated with a continuous wave (CW) electromagnetic single frequency waveform, the reflected energy is shifted in frequency in proportion to the target's velocity (V) and inversely as the carrier's wavelength (C/f), where f is the carrier frequency. The shift in frequency f.sub.d, is called the Doppler frequency. If a single, narrow frequency is used, the shift in frequency provides an indication of the movement of the target with respect to the radar receiver tracking the target. If a multioctave, wideband radar system is used, the frequency of the target echo return signal might not be perceived as a shifted frequency. More specifically, the shifted frequency of the echo return of a moving target can be close to one of the transmitted frequencies of the wideband radar transmitter, and therefore there exists an ambiguity in the Range-Doppler frequency of the return signal.
Another source of ambiguity is the "velocity walk" phenomenon of the radar return signals of accelerating targets. The velocity of an accelerating target changes over an interval of time. Therefore the frequency of the target echo return signal is not only shifted, but has both a shifted frequency and a frequency that is changing in real time. Uncompensated velocity walk phenomena produce a "smearing" of the Doppler offset over the coherent processing interval and are an additional source of ambiguity.
The task of resolving the Range-Doppler frequency ambiguities experienced by ultra-high wideband radar tracking systems is alleviated, to some extent by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference;
U.S. Pat. No. 3,378,838 issued to Romano;
U.S. Pat. No. 3,500,400 issued to Woerrlein
U.S. Pat. No. 3,969,725 issued to Couvillon et al.,
U.S. Pat. No. 4,096,478 issued to Chavez,
U.S. Pat. No. 4,568,939 issued to Grau; and
U.S. Pat. No. 4,689,627 issued to Lee et al.
The Couvillon U.S. Pat. No. 3,969,725 for improved distance measuring equipment consist of an airborne linear chirp (LFM) interrogator transmitter, and a correlating (or matched filter) receiver at a fixed ground station. A LFM pulse burst pair (two pulses) is transmitted, in one of four combinations, up ramp/up ramp, up ramp/down ramp, down ramp/up ramp, or down ramp/down ramp, to reduce range-doppler ambiguities. The pulse burst waveform is received at the ground station, and decoded. A conventional pulse gated RF burst pair is reradiated by the transponder, received by the airborne unit, processed and distance information is produced the Convillon.
The Lee et al. patent disclolses an ultra-wideband radar system.
The Chavex patent employs a chirped pulse pair for eliminating or reducing range ambiguties, in a fashion very similar to the Couvillon patent.
The Romano U.S. Pat. No. 3,378,838 and Woerrlein U.S. Pat. No. 3,500,400 patent employ pulse burst pairs in the elimination of Range-Doppler ambiguities, where pulse to pulse frequency hopping (within a pulse burst) is utilized to provide an additional signal processing discriminant, (to reduce ambiguities). As in the Couvillon and Chavez patents, multiple pulse processing is required to resolve ambiguities, in stark contrast to the subject invention.
While the above-cited references are instructive, the need remains to provide an unambiguous Range-Doppler radar processing system. The present invention is intended to satisfy that need.